credentials: Use net.SplitHostPort to safely parse IPv6 authorities in ClientHandshake

[jpbetz]

Correctly split authorities with IPv6 addresses and no port, e.g. [::1].

[jpbetz]

cc @liggitt

[easwars]

Thanks for your contribution.

[jpbetz]

Looks like Travis CI does not support IPv6: "Failed to listen: listen tcp [::1]:0: bind: cannot assign requested address" (https://travis-ci.org/grpc/grpc-go/jobs/595347032). I'll comment out the IPv6 test case with a comment explaining.

[liggitt]

Or detect lack of ipv6 support and skip the test

[easwars]

Something like this should suffice. Thanks.

	if err != nil {
		if strings.Contains(err.Error(), "bind: cannot assign requested address") {
			t.Skip("missing IPv6 support")
		}